1. Field of the Invention
This invention relates to controlling fluid flow rates to two separate fluid sinks, and more particularly, to means and method for controlling the flow rate of condensate to a feedwater heater and main condenser in a heat/power conversion cycle.
2. Description of the Prior Art
Large central station power generation facilities typically utilize heat-power cycles in which a working fluid such as water is vaporized, expanded through a power turbine coupled to an electrical generator, condensed at the exhaust from the turbine in a condenser, and pressurized prior to vaporizing it again. At each of many locations in the heat-power cycle condensed water is routed to a heat-recovery device such as a feedwater heater. In some cases, however, when the flow rate of the condensate became excessive or the receiving feedwater heater was out of service, the excess condensate was often routed to other fluid utilizing devices such as other feedwater heaters of the condenser which can accept the additional, excess flow without adverse operational consequences.
For power cycles having controlled nuclear fission as the heat source, moisture separator reheater apparatus are commonly used to remove water from steam which has been partially expanded through the power turbine. The partially expanded steam is removed from one section of the turbine, transmitted through moisture separators which extract moisture from the steam, passed across the outside tube surfaces of a reheating tube bundle(s) to be reheated, and returned to a lower pressure section of the turbine. Higher temperature, reheating steam taken from a steam generator or other source is routed through the tubes of such reheating tube bundle(s) where it gives up a portion of its heat to the partially expanded steam and condenses. The partially expanded steam flowing on the outside of the reheating tube bundle(s) is reheated by the condensing, higher temperature steam circulated through the tubes of such tube bundle(s). When, multiple reheating tube bundles are used to reheat the partially expanded steam in stages to maximize the thermodynamic efficiency of the reheating process, each bundle is supplied with steam at a temperature different from the temperature supplied to other bundles. Each bundle is characterized by the steam temperature supplied to it with the progressively higher temperature tube bundles being arranged in the reheated steam's normal flow direction through the shell.
Condensate from the tube side of each tube bundle and the separated moisture from the partially expanded steam is removed from the tube side and shell side, respectively, and drained to a common or separate fluid sink(s) such as a feedwater heater(s) or condenser. Thermodynamically, it is most desirable to cascade any elevated temperature fluid to cycle heat recovery apparatus such as a feedwater heater rather than a cycle heat rejecting apparatus such as a condenser since retention of heat within the cycle increases the cycle's efficiency and reduces its operating cost. Thus, it is desirable to transmit the maximum possible percentage of condensate flow to heat recovery devices such as feedwater heaters. However, such heat recovery devices can typically accommodate limited flow rates before their performance is adversely affected. In cases of excess condensate flow and in cases where the heat recovery devices are out-of-service, the flow must be routed to less efficient heat recovery or heat rejection devices.
To facilitate control of condensate drainage from tube bundles, moisture separators, or other sources, the condensate is commonly routed through a relatively small drain tank. After steady state flow is achieved, increased condensate flow rates are reflected in increasing fluid heights in the drain tank. A signal indicative of the fluid level in the drain tank has typically been generated by a first controller apparatus and that signal has actuated a modulating valve to cause it to regulate the fluid flow rate from the drain tank. As the level in the drain tank increased above a desired, normal level, a first modulating valve for regulating fluid flow to the most efficient fluid sink continued to open as a result of the signal's level indication.
When the fluid flow rate to the drain tank surpassed the maximum flow rate transmissible by the first valve in its unrestricted flow position, the level in the drain tank continued to rise until a designated high level was reached. At such time, a second controller generated a signal which was trasmitted to a second modulating valve to cause it to open until the drain tank fluid level was reduced below the designated high level. Use of such separate full-range level controllers on horizontal, small diameter drain tanks occasionally presented difficulties when the separation between the normal and high fluid levels was insufficient. As a result of such insufficient level separation, the two level controllers interacted in modulating their respective valves and caused control system instability. Furthermore, failure of the first full-range controller caused all flow to be routed through the second modulating valve and thus reduced the efficiency of the power cycle. Failure of the second full-range controller necessitated the routing of all condensate through the first modulating valve. If, during exclusive condensate routing through the first valve, the condensate flow increased beyond the flow rate transmissible through the first valve, a costly and time-consuming shutdown of the entire power generation cycle would have been necessary to avoid consequences resulting from condensate backup into the moisture separator reheaters.
The aforementioned controller interaction problem was overcome by utilizing a split-range controller to sequentially open the first and second modulating valves by generating and transmitting a signal indicative of the drain tank's level to both modulating valves. The second valve was designed to respond to signals indicative of fluid levels greater than the designated high level. Such control system eliminated the interaction problem previously described, but provided no redundancy in the case of controller failure. Failure of such split-range controller may have necessitated the aforementioned unscheduled shutdown of the entire power generation cycle.
Desirable condensate drainage control system features include redundant, non-interacting controllers, sequential condensate distribution to two fluid sinks, and remote or automatic switching capability between first and second non-interacting controllers. Such features were not concurrently available on any prior art condensate drainage control system.